Polymer foam plank with densified skin, method and apparatus for producing a densified skin on a foam plank

ABSTRACT

A method and apparatus for making a densified skin polymer foam plank comprising providing a polymer foam having a plurality of cells and heating an outer surface of the foam at a heating station. The heating step collapses and melts the cells adjacent the outer surface to form the densified skin having a density greater than a density of the foam before the heating step. Additionally, the method and apparatus may further include extruding the polymer foam from an extruder.

FIELD OF THE INVENTION

The present invention relates generally to polymer foam planks and, moreparticularly, to a foam plank with a densified skin and a method and anapparatus for making the densified skin foam plank.

BACKGROUND OF THE INVENTION

Foam structures are useful as protective packaging. Protective packagingsystems have employed foam structures to suspend a product in the centerof a container during shipping and storage. For example, foam end capsfit on opposite ends of a product such as a computer, printer, computermonitor, medical monitoring device or other fragile electronicequipment, to protect the product from shock and vibration damage. Ingeneral, protective packaging foams are low density polymeric materialswith good physical properties capable of supporting the product weightwithout excess deformations during package transit and storage. Theexact foam density required for a particular application depends on thefoam compression properties, shear properties, shock mitigationproperties during package drops, creep properties, buckling limits, andthickness resiliency with multiple drops.

Laminated foam structures have been developed that provide enhanced foamphysical properties and/or simplify the creation of fabricated end capsfor protective packaging systems. Some laminated foam structurescomprise a low density foam core with one or more layers, and one ormore skins of high density foam laminated to the core. U.S. Pat. Nos.5,876,813 and 5,882,776 describe examples of such laminated foamstructures. In addition, other laminated foam structures may comprise alow density foam core with one or more layers, and one or more skins ofthin polymeric film or polymeric sheet. These laminated foam structuresprovide the desired property enhancements.

Conventional lamination techniques produce the laminated foamstructures, including the techniques of bonding the layers using heat,film, or applied adhesives. One problem with the laminated foamstructures is their cost and inefficiency of production. The laminationprocess for the laminated foam structures first requires the productionand stocking of various rolled foam sheet materials or individual foamplanks having different properties, densities, dimensions, and colors.These foam materials, after a curing time, are then laminated onseparate lamination equipment to form the finished product. Laminatedfoam structures with polymeric films and/or sheets also require aseparate production process. Often, polymeric films and sheets areproduced off-site from the foam and laminate production location.Therefore, film rolls and sheets must be transported to the laminationplant. Another problem with the conventional lamination techniques isthat an inventory of component foams, films and/or sheets must bemaintained to meet manufacturing scheduling. Furthermore, all componentmaterials must be inspected and certified to meet quality and productspecifications on an individual basis before being used at thelamination production stage.

Thus, there is a need to develop foam structures having enhancedproperties. There is also a need to manufacture these foam structureswith a relatively simple production process that does not have theinefficiencies and complexities associated with the conventionallaminated product production processes. The present invention isdirected at satisfying these needs.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, there isprovided a method for making a densified skin polymer foam plank. Themethod comprises providing a polymer foam having a plurality of cellsand heating an outer surface of the foam. The heating step collapses andmelts the cells adjacent the outer surface to form the densified skinhaving a density greater than a density of the foam before the heatingstep. Additionally, the method further includes a step of extruding thepolymer foam from an extruder. Prior to extruding the foam, the methodmay further include the steps of mixing and melting a polymer mixturecomprising at least 50 weight percent low density polyethylene, mixingthe mixture with one or more blowing agents, cooling and pressurizingthe mixture.

In accordance with another aspect of the present invention, there isprovided an apparatus for making a densified skin foam plank. Theapparatus comprises a heating station capable of heating an outersurface of an ethylene polymer foam plank to form a densified skin. Thedensified skin has a density at least ten times the density of theethylene polymer foam before the heating. The heating station maycomprise hot air knifes. The apparatus may further include an extrudercapable of producing the ethylene polymer foam out of an extrusion die.The extruder may be in-line with the heating station to continuously orintermittently provide the ethylene polymer foam to the heating station.

In accordance with a further aspect of the present invention, there isprovided a densified skin ethylene polymer foam plank. The plankcomprises a low density ethylene polymer foam having dimensionallystable closed cells. The plank also includes a densified skin on a topsurface of the foam. The densified skin comprises melted and collapsedfoam cells formed by heating the top surface of the foam.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other advantages of the invention will become apparentupon reading the following detailed description and upon reference tothe drawings in which:

FIG. 1 is a cross section of the foam plank having a densified skinembodying the present invention;

FIG. 2 is a cross section of a foam plank having a pair of densifiedskins;

FIG. 3 is a cross section of the foam plank of FIG. 2 having a hinge;

FIG. 4 is a simplified process diagram of the method for producing thefoam plank of FIG. 2; and

FIG. 5 is a side view of one embodiment of a heating station.

While the invention is susceptible to various modifications andalternative forms, specific embodiments have been shown by way ofexample in the drawings and will be described in detail herein. However,it should be understood that the invention is not intended to be limitedto the particular forms disclosed. Rather, the invention is to cover allmodifications, equivalents, and alternatives falling within the spiritand scope of the invention as defined by the appended claims.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Turning now to the drawings and referring initially to FIG. 1, there isdepicted a polymer foam plank 10 according to the present invention. Thefoam plank 10 includes a core 12 and a densified skin 14. In oneembodiment, the core 12 comprises a stable closed cell ethylene polymerfoam with a density between 0.9 and 5.0 lb/ft³. In an anotherembodiment, the core 12 may comprise more than one foam layer with allof the layers having approximately equal densities. The densified skin14 comprises the same polymer foam as the core 12, but the skin 14 has adensity greater than the density of the core 12. The densified skin 14typically will have a density sufficient to provide the desired physicalproperties described below. For example, the density of the skin 14 mayhave a range of a density slightly greater than the density of the foamcore 12 to a density equal to the density of the solid polymer fromwhich the foam plank is produced. In some embodiments, the density ofthe skin 14 is at least ten times the density of the foam core 12. Thedensified skin 14 is formed by exposing a foam plank or sheet to a heatsource, as will be described in detail in connection with FIG. 4. Forexample, exposing the foam core to heat of sufficient temperature andduration will densify the foam nearest to the heat application throughcell melting and cell collapse resulting in the foam plank 10 of thepresent invention.

In one embodiment, the foam plank 10 may have a thickness greater than1.0 inches. The thickness of the densified skin 14 may range from about12.5 mils to 100 mils. In other embodiments, the core 12 and skin 14 mayhave a wide range of possible thicknesses dependent upon the initialamount of the foam core 12 thickness that is reduced by the heatexposure into the densified skin 14 and the final density of thedensified skin 14. In most embodiments of the present invention, thefoam plank 10 is less than 4 inches in thickness with the densified skin14 having an approximate thickness range of between 12.5 and 30 mils.Additionally, the density of the densified skin 14 is typically greaterthan 50% of the solid polymer density from which the foam core 10 isproduced.

FIG. 2 depicts a polymer foam plank 20 according to another embodimentof the present invention. In this embodiment, the foam plank 20 includesa core 22 sandwiched between an top densified skin 24 and a bottomdensified skin 26. Similar to the embodiment depicted in FIG. 1, thecore 22 comprises a stable closed cell ethylene polymer foam having adensity between 0.9 and 5.0 lb/ft³. The skins 24 and 26 comprise thesame polymer foam as the core 22 but the skins 24 and 26 have a densitygreater than the density of the core 22. The skins 24 and 26 are formedby exposing the plank to a heat source of sufficient temperature andduration to densify the foam nearest the heat application, as will bedescribed in detail in connection with FIG. 4. The densified skins 24and 26 typically will have a density sufficient to provide the desiredphysical properties described below. For example, the density of theskins 24 and 26 may have a range of a density slightly greater than thedensity of the foam core 22 to a density equal to the density of thesolid polymer from which the foam plank is produced. In someembodiments, the density of the skins 24 and 26 is at least ten timesthe density of the foam core 22. In one embodiment, the foam plank 20may have a thickness greater than 1.0 inches. The thickness of thedensified skins 24 and 26 may range from about 12.5 mils to 100 mils. Inother embodiments, the core 22 and skins 24 and 26 may have a wide rangeof possible thicknesses dependent upon the initial amount of the foamcore 22 thickness which is reduced by the heat exposure into thedensified skins 24 and 26 and the final density of the densified skins24 and 26. In most embodiments of the present invention, the foam plank20 is less than 4 inches in thickness, the densified skins 24 and 26 arebetween 12.5 and 30 mils in thickness, and the density of the densifiedskins is greater than 50% of the solid polymer density from which thefoam core 20 is produced.

The densified skin foam planks 10 and 20 have enhanced physicalproperties as compared to foam planks without densified skins. Thedensified skin foam planks 10 and 20 generally provide increasedcompression strength and modulus, improved cushioning performance,increased tear resistance, improved thermal stability, increased tensiletoughness, and decreased compression creep compared to foam plankswithout densified skins. The low density core 12 and 22 contributes to alow total weight of the plank 10 and 20, while the high density skins14, 24 and 26 provide aesthetic improvement and improved physicalproperties. Additional benefits of densified skin foam plank 10 and 20are a more durable and smoother surface, improved wear resistance,increased flexural stiffness, and improved die cutting characteristics.

The densified skin foam plank 10 and 20 with enhanced properties isideal for protective packaging. In addition, the densified skin foamplank 10 and 20 may be used in exercise equipment, such as gym mats,water sports, such as bodyboards, and construction applications. In oneembodiment, the densified foam plank 20 may be die cut for protectivepackaging applications. As illustrated in FIG. 3, a densified foam plank30 includes a slit 32 extending through the bottom skin 26 and the core22 but not through the top skin 24 forming a hinge 34.

Because of the densified skin layer 24, the hinge 34 has good mechanicalstrength in tension and shear, thereby having utility in collapsiblepacking members. In such collapsible packing systems, a densified foamplank 20 is die cut such that one or more portions of the plank canrotate from a flat storage position around the hinge 34. Once rotated,the individual functional portions of the die cut foam plank form 30 anintegral end cap suitable for the protective packaging of electronic andother consumer goods. Therefore, die cutting and slitting the densifiedfoam plank 30 provides a wide variety of protective packagingapplications.

In other embodiments, the slit 32 may only extend partially through thecore 22 or the slit may extend partially through the top skin 24. Diecutting or slitting exactly to the densified top skin 24 is generallypreferred. This “slit to skin” technique provides a design guide forcutting blades to ensure repeated production and hinge performance. Inaddition, the “slit to skin” technique provides a neater appearance ofthe die cut part when folded and an improved resistance to hinge tearcaused by crack propagation emanating from the foam core 22 during adynamic drop.

The densified skin foam planks 10 and 20 of FIGS. 1 and 2 are producedusing a method and apparatus according to the present invention. FIG. 4illustrates a simplified process diagram of the method and apparatus forproducing densified foam planks. First, low density closed cellpolyolefinic foam is extruded through a die 42 utilizing either aconventional continuous foaming process or a conventionalsemi-continuous accumulating extrusion system as known to one skilled inthe art.

In the conventional polyolefinic foam extrusion processes, pellets ofthermoplastic resin are dry blended with a solid phase nucleating agentand other miscellaneous additives. The blend is then melted in a heatedextruder 40 where the resulting polymer mixture is held at a hightemperature and pressure. A physical blowing agent and a permeationmodifier agent are then added to the pressurized melted material. Theblowing agent generally liquefies and dissolves into the polymer meltwithin the extruder and will vaporize at die melt temperatures andatmospheric pressure. The permeation modifier agent is usually an esterof a fatty acid having a chain of 16-22 carbon atoms. The permeationmodifier agent prevents the collapse of the resulting foam structureover time. The blowing agent and permeation modifier agent are mixedwith the melted plastic and nucleating agent and the combination issubsequently cooled to an extrusion temperature suitable for foaming.The cooled polymer melt is pushed through a die by the pressuregradient. When released to atmospheric pressure, the dissolved blowingagent vaporizes and expands to form bubbles of gas at the nucleatingsites provided by the uniformly dispersed nucleating agent particles.The extrusion rate through the die 42 may exceed 1000 lb/hr to producean expanded and dimensionally stable closed cell ethylene polymer foam44. The foam 44 has a density between 0.9 and 5.0 lb/ft³ and a thicknessof at least 1.0 inch.

Once the foam 44 has been extruded from the die 42, the foam 44 passesto heating stations 46 and 48 as depicted in FIG. 4. The heating sourceof the heating stations 46 and 48 is heated air, infrared heaters, orother conventionally known heating sources that can provide the desiredskin densification. Combinations of various heating devices also workunder the present invention. In the illustrated embodiment of FIG. 4,the foam plank 44 is heated on both sides. However, the foam plank 44can be optionally exposed to heat on only one side in other embodiments.

In the embodiment illustrated in FIG. 5, the heating stations 46 and 48include a series of one or more temperature controlled heated air knives60, 62, 64 and 66. Rollers 68, 72 and 76 form a pressure regulated nippoint with other rollers 70, 74 and 78, or a driven belt, and arepositioned at the in-feed and/or out-feed of the heating stations 46 and48, and optionally, between any two or more individual heated airknives. In one embodiment, these rollers 68-78 are temperaturecontrolled to prevent the heated densified foam skin from sticking tothe surfaces of the rollers 68-78.

Prior to entering the heating stations 46 and 48, the foam 44 is allowedto stabilize for a period of time. For stabilization, the foam 44 isheld for approximately 30 seconds and preferably for 2 to 5 minutesprior to exposure to the heating stations 46 and 48. This delay allowsfor good stabilization of the foam 44 following extrusion but maintainsthe plank core temperature above ambient conditions allowing the skindensification process to be more efficient. In other embodiments, theextruded foam plank 44 may age for an extended time period prior toexposure to the heating stations 46 and 48. For the aged foam planks,the foam 44 can be fed to an off-line heating station assembly similarto that described above in conjunction with FIG. 4.

The speed of the foam 44 passing though the heating stations 46 and 48is regulated to control the residence time exposure to the heatingsource. When the foam 44 is exposed to heat at the heating stations 46and 48 the outermost cellular region of the foam 44 densifies to form adensified skin plank 50. During the densification process, the outermostcells of the foam 44 soften, melt and collapse to form an increasinglythick layer of densified foam and/or partially voided solid polymer atthe surface of the foam plank 44. Increasing the time duration andexposure temperature can, to a point, increase the thickness of theskins 54 and 56 as well as their density. Increasing the thickness anddensity of the skins 54 and 56 is limited by the heat transfer ratethrough the increasingly thick skin layer to the cellular foam below.For this reason, densification of foam plank 44 whose core is stillwarm, such as that produced with the foam extrusion process in line withthe heating stations 46 and 48 as shown in FIG. 4, is generallypreferred because the heat transfer is maximized. The temperaturecontrolled rollers 68-78 and their pressure regulated nip force alsoassist in controlling the density and smoothness of the densified skin.

Once the foam 44 has been exposed to the heating stations 46 and 48, thefoam is a densified skin foam plank 50. The plank 50 includes a lowdensity core 52 having a density equivalent to the density of theextruded foam 44. The plank 50 also includes top and bottom skins 54 and56 having higher densities than the density of the core 52. In oneembodiment, the skins 54 and 56 have densities between ten times thedensity of the foam core 52 and the density of the solid polymer fromwhich the plank 44 was produced. The thickness of the top and bottomskins 54 and 56 range from 12.5 to 100 mils. The plank 50 is similar tothe plank 30 of FIG. 2. To form a plank having only one densified skinas shown in FIG. 1, either the upper heating station 46 or lower heatingstation 48 but not both densifies one of the two skin surfaces of thefoam plank 44.

In one embodiment, after the heating stations 46 and 48 have densifiedthe skins 54 and 56, the plank 50 moves to a cutting station 58. Thecutting station 58 may be part of the heating station assembly orcontained as a separate unit operation. At the cutting station 58, theplank 50 may be completely cut through and divided into appropriatelengths and widths. Additionally, the cutting station 58 may slit theplank 50 to form a hinge as depicted in FIG. 3 leaving the top skin 54intact while cutting the core 52 and bottom skin 56. The resultingdensified plank 50 also may be die cut into appropriate protectivepackaging.

In this description, the term “polyolefinic resin (polymer or material)”is meant to include polymers of linear or branched C₂ to C₈ hydrocarbonmolecules that contain one double bond in its structure includingalkenes such as ethene, propene, 1-butene, 2-butene, 1-pentene,2-pentene, and 3-pentene. Generally preferred polyolefinic materialsinclude polymers of ethene, which are commonly known as polyethylene.Broadly, the invention involves a method for preparing closed cellethylene polymer foams with densities between 0.9 and 5.0 lb/ft³densities, thicknesses greater than 1.0 inch, with one or more densifiedskins formed from the closed cell ethylene polymer foam. A closed cellfoam can be defined as one with fewer than 15% open cells, as determinedby water absorption tests or air pycnometer measurements. The densifiedskins of the ethylene polymer foam typically have densities that rangefrom ten times the density of the ethylene polymer foam core to slightlyless than the density of the solid ethylene polymer from which the foamcore was produced. Furthermore, the densified skins are from 12.5 to 100mils in thickness, dependent upon the density of the densified skins andthe amount of original ethylene polymer foam thickness densified intodensified skins.

The ethylene polymer resin that is used to produce the foams of thepresent invention can be those obtained by polymerizing ethylene, orpolymerizing ethylene with other aliphatic polyolefins such aspropylene, 1-butene, 1-pentene, 3-methyl-1-butene, 4 methyl-1-pentene, 4methyl-1-hexene, or 5-methyl-1-hexene alone, or with mixtures thereof,or with various other polymerizable monomers. Ethylene polymers includehomopolymers of ethylene and copolymers of ethylene and otherethylenically unsaturated monomers having from three to about eightcarbon atoms including propylene, butenes, pentenes, hexenes, and thelike. The copolymers can include other monomers compatible withethylene. Generally preferred are medium density polyethylene (MDPE),low density polyethylene (LDPE), and linear low density polyethylene(LLDPE). Such polyethylenes are described in the Kirk OthmerEncyclopedia of Chemical Technology, Third Ed., Vol. 16, pages 385, 420;the Modern Plastics Encyclopedia (1986-87), pages 52-63; and theEncyclopedia of Polymer Science and Technology, Vol. 7, page 610.

The term “ethylene polymer (resin or material)”, as used herein, ismeant to include not only homopolymers of ethylene, but also ethylenecopolymers composed of at least 50 mole percent (preferably at least 70mole percent) of an ethylene unit and a minor portion of a monomercopolymerizable with ethylene. In addition, the term “ethylene polymer”includes blends of at least 50 percent by weight of an ethylenehomopolymer with another polymer or blend of polymers.

Generally preferred ethylene polymers include non-crosslinked, lowdensity polyethylene having a density of about 0.915 to 0.930 kg/m³ anda melt flow rate in the range of about 0.1 to 6.0 grams per 10 minutes,as measured using ASTM D1238 at 190° C. and 689.5 kPa load. Morepreferred in the present invention are blends of low densitypolyethylene (LDPE), recycled LDPE and/or recycled ethylene polymer,other ethylene polymers and copolymers, and other non-olefinichompolymers and copolymers.

The recycled LDPE in the mixture may be post consumer or a stream ofLDPE foam from a commercial fabricator of protective packaging. WhileLDPE recycle is generally preferred, recycle containing whole or partialfractions of other ethylene polymers is also permissible. The LDPErecycle in the present invention may range from 0 to 100 percent byweight of the resin composition used to produce a densified skin plank.

Permissible copolymer content may range from 0 to 40 percent by weightof the total resin composition and is selected from a list including,but not limited to, saturated and unsaturated styrene-butadiene randomand block copolymers rubbers, ethylene vinyl acetate (EVA), ethyleneacrylic acid (EAA), ethylene methacrylic acid (EMAA), ethylene vinylalcohol (EVOH), ethylene propylene diene monomer copolymer rubbers(EPDM), ethylene-propylene copolymers, styrene-ethylene copolymers andinterpolymers, ethylene ethyl acrylate (EEA) and ethylene methylacrylate (EMA).

Formulation blends with other homopolymers and copolymers are alsoacceptable as long as the total LDPE content exceeds 50 weight percentof the total resin composition. Acceptable homopolymers and copolymersinclude, but are not limited to polystyrene, polyamides, polyolefinelastomers and plastomers, polypropylene, medium density polyethylene(MDPE), high density polyethylene (HDPE), linear low densitypolyethylene (LLDPE), and olefinic ionomers.

The nucleating agent, or cell size control agent, can be any convenientor useful nucleating agent(s). The cell size control agent is preferablyused in amounts of 0.1 to 2.0 weight percent, depending upon the desiredcell size and based upon the weight of the polyolefinic resin. Examplesof the cell size control agent are inorganic materials such as clay,talc, silica, and diatomaceous earth. Other examples include organiccell size control agents which decompose or react when heated in theextruder to evolve nitrogen or carbon dioxide gas, such asazodicarbonamide, hydrocerol, etc. Generally preferred nucleating agentsare talc, silica, or a stoichiometric mixture of citric acid and sodiumbicarbonate. Mixtures of cell size control agents may be used. Otheradditive concentrates and materials may also be added to the extruderwith the ethylene polymer feed including antioxidants, color pigments,UV additives, and antistatic agents.

Permeation modifiers may be used in the foamable composition of theinvention to prevent collapse of the cellular structure within the firsttwenty-four hours following extrusion. Permeation modifiers are alsocalled aging modifiers in some polyolefinic extrusion art. Generallypreferred aging modifiers include fatty acid esters such as glycerolmonostearate.

The physical blowing agents used for the present invention includeorganic and inorganic blowing agents. Permissible organic blowing agentsincluded C₂ to C₅ aliphatic hydrocarbons, such as ethane, propane,n-butane, isobutane, n-pentane, isopentane, and neo-pentane. Permissibleorganic blowing agents also include halogenated hydrocarbons includingHFC's HCFC's and CFC's. Examples of the halogenated hydrocarbon blowingagents include 1,1,1,2,2-pentafluoroethane (HFC-125),1,1,1,2-tetraflouroethane (HFC-134a), 1-chloro-1,2-difluoroethane(HCFC-142b), 1,1,1-trifluoroethane (HFC-143a), and 1,1-difluoroethane(HFC-152a). Permissible inorganic blowing agents include inorganic andinert gases at room temperature such as nitrogen, argon, and carbondioxide.

Referring to FIG. 4, in one embodiment the foam plank 44 can be producedfrom a process comprising the steps of; feeding an ethylene polymer intoan extruder 40; adding a nucleating agent to the resin feed; optionallyadding a permeation modifier to the resin feed; optionally adding otheradditives such as color pigments, etc. to the resin feed; plasticatingthe mixture in an extruder, 40, to form a polymeric melt; incorporatingan organic or inorganic blowing agent, or combinations thereof, into thepolymer melt; optionally injecting a liquefied permeation modifier intothe polymer melt; uniformly mixing and cooling the foamable compositionto a temperature effective for the expansion of the ethylene polymerfoam; and extruding or ejecting the foamable composition through a die42 at a sufficient high rate to form a closed cell polyolefinic foam.

The present invention may implement a continuous plank extrusion processor an intermittent accumulating extrusion process. The foamablecomposition can be used in an extrusion process operated on a continuousbasis using a conventional extruder system. The continuous process forpolyolefinic foams can produce foam of any thickness, but generally lessthan 4 inches in thickness at a 24-inch width. The intermittentaccumulating extrusion process is generally used for large cross-sectionpolyolefinic foams with thicknesses greater than 1.0 inches. The twoprocesses have similar extrusion conditions, but differ slightly inpreferred composition. In both processes, the foam plank 44 is producedby extruding the foamable composition through a die 42 at instantaneousrates greater than 1000 lb/hr. The foam plank 44 used in the presentinvention has a density between 0.9 and 5.0 lb/ft³ and a thicknessgreater than 1.0 inch, but generally less than 4.0 inches.

Referring to FIG. 4, in one embodiment the foam plank 44 is made into adensified plank 50 by a process comprising the steps of: conveying theplank 44 to heating stations 46 and 48 following a minimum stabilizationtime period after extrusion; exposing the foam plank 44 to a single ormulti-zone temperature controlled heat source on at least one side for aspecified time duration controlled by regulating the speed of the plankthrough the heating stations; passing the foam plank 44 under one ormore temperature controlled pressure regulated nip rollers in closeproximity to the heating zones; and optionally conveying the densifiedskin plank 50 to a cutting station 58 that is separate from, directlyattached to, or integral with the heating station assembly.

Referring to FIG. 4, the heating stations 46 and 48 may consist of asingle or multiple temperature controlled heating source zones. Theheating source is heated air from temperature controlled hot air knives,infrared heaters of sufficient watt density, or other conventionallyknown heating sources that can provide the desired skin densification.Combinations of various heating devices can be used within a given heatsource zone, or as a distinct heat source zone unto itself.

In one embodiment of the present invention, the heating source zones aretemperature controlled hot air knives 60-66. The discharge knife nozzledesign provide a uniform exit flow velocity across the width of theknife. The nozzle discharge temperature is controlled at a temperaturegreater than 400° F. The autoignition temperature of the blowing agentused to produce the foam, if the blowing agent is flammable, limits themaximum nozzle discharge temperature. The width of the nozzle is equalto or greater than the width of the foam plank 44 whose skin is beingdensified. It is also acceptable to position nozzles, whose width isless than the width of the foam plank, side by side to provide thedesired plank width coverage. The distance of the heating nozzle fromthe foam plank 44 being densified can vary depending on the volumetricflow rate and exit velocity of the hot air through the nozzle. In oneembodiment of the present invention, the distance of the nozzle from thefoam plank 44 being densified is less than 1.0 inch.

As described above for one embodiment of the heating stations, rollers68-78 form a pressure regulated nip point with other rollers or a drivenconveyor belt and are positioned at the in-feed and/or out-feed of theheating station assembly. Additionally, rollers 68-78 may form for thenip point between any two or more individual heated air knives 60-66 orother heating source zones. In one embodiment of the present invention,these rollers 68-78 are controlled at a temperature below the meltingpoint of the ethylene polymer used to produce the densified plank 50.The roller nip pressure is regulated to provide sufficient normal forceto the foam plank 44 to improve the resulting surface smoothness of thedensified foam plank 50 and to assist in the skin densification of thefoam plank without crushing or distorting the foam core 52.

Prior to entering the heating stations 46 and 48, the freshly extrudedfoam plank 44 is allowed to stabilize for a period of time. In oneembodiment, the foam 44 is held for a minimum of 30 seconds prior toexposure to the heating stations 46 and 48. In another embodiment, thestabilization time is between 2 and 5 minutes after the foam plank exitsthe die. For continuously extruded foam plank, this is accomplished byproviding a conveying length of sufficient distance to give the desiredresidence time between the die exit and entrance to the heating stations46 and 48. For the intermittent accumulating extrusion system, thestabilization time is provided by holding the ejected foam plank 44 in aforming table for a time period up to the cycle time of the plankejection.

In another embodiment of the present invention, the extrusion process 40for the foam plank 44 is separated off-line from the heating stations 46and 48. In this configuration, the freshly extruded foam plank 44 isaged off-line anywhere from 5 minutes to many weeks. The aged plank isthen fed into the heating station assembly 46 and 48 in a separate unitoperation step. Generally, the separated process is less preferred dueto the extra handling of the foam plank. In addition, the retained heatin the foam core 52 is lost resulting in a less efficient densificationprocess. As discussed above, feeding the foam plank 44 directly to theheating stations 46 and 48 has been found to improve the efficiency ofthe skin densification and increase the maximum obtainable skinthickness.

Skin densification of the foam plank 44 is primarily determined by heatinput to the plank, the nip force used to compress the foam nearest thesurface after softening by the heat, and the heat transfer rate throughthe foam and densified skin as the skin is densified. In one embodimentof the present invention, heat input is controlled by the temperatureand volumetric flow rate of the hot air supplied through the nozzles,the number of nozzles utilized, and the axial speed of the plank as itpasses under each nozzle or heating zone. Typical line speeds are 5 to40 ft/min, depending on the extrusion rate, plank dimensions, and foamdensity being produced. In one embodiment of the present inventionutilizing an intermittent accumulating extrusion process, the foam plankvelocity through the heating stations 46 and 48 is controlled at thelowest speed possible that allows the plank on the forming table to exitjust before the next foam plank is ejected from the accumulatingextrusion system.

The heat transfer rate through the thickening densified skin layers 54and 56 becomes the limiting factor for determining the final skinthickness of the densified skin foam plank 50. A limit is reached afterwhich it is no longer possible to heat the foam core 52 to createadditional skin thickness. The heat transfer rate is determined by thedensified skin thickness, the densified skin density, and the initialdensity of the foam plank 44. One observed practical upper limit forskin densification is that the maximum thickness loss of the foam plank44 during densification is 1.0 inch per side heated for a 2.0 lb/ft³initial foam density. Typically, the total thickness loss of the foamplank 44 during densification is between 0.25 and 0.5 inch per sideheated.

Upon exiting the heating stations 46 and 48, the foam is a densifiedskin foam plank 50. The plank 50 includes a low density core 52 having adensity equivalent to the density of the extruded foam 44. In addition,the densified skin foam plank 50 includes densified skins 54 and 56 onthe top and bottom, or optionally, a densified skin on one side only.The skins 54 and 56 have higher densities than the density of the core52. In one embodiment, the skins 54 and 56 have densities between tentimes the density of the foam core 52 and the density of the solidethylene polymer from which the plank 44 was produced. The densifiedskin density can be determined by buoyancy force during immersion inwater, or by measuring the weight of a given area and thickness ofdensified skin. The thickness of the densified skins 54 and/or 56 rangefrom 12.5 to 100 mils. Skin thickness is measured by skiving thedensified skin away from the foam core and using a ratchet styleMitutoyo digital micrometer with a ¼″ flat foot, or equivalentalternative thickness measuring device. For reference purposes, the foamplank 44 is extruded with thin skins. The maximum skin thicknessobtained by varying extrusion conditions and the temperature of the die42 has been 7 mils. Generally, the skin thickness of the extruded foamplank 44 is less than or equal to 5 mils.

The densified skin foam plank 50 has enhanced properties in comparisonto planks without densified skins. An illustrative comparison ofproperties for a plank sample having a densified skin on one sidecompared to a plank sample having no densified skins is shown in Table 1below. Details for the preparation of the densified skin plank sampleare given in Example 1 of the present invention.

TABLE 1 ILLUSTRATIVE PROPERTY COMPARISON Example 1 With No. of StandardExample 1 Without No. of Standard TEST PERFORMED Densified Skins SamplesDeviation Densified Skins Samples Deviation Density (pcf) 2.28 3 0.0362.15 3 0.030 Cell Count (cpi) Near Densified Skin CMD 21.3 3 1.155 N/AMD 21.3 3 1.155 N/A Middle CMD 21.3 3 1.155 19.3 3 1.155 MD 21.3 3 1.15523.3 3 1.155 Compression Strengths (psi) @ 25% Deflection 50% DeflectionVertical 7.70 3 0.099 5.68 3 0.231 15.50 3 0.244 12.20 3 0.509Horizontal 8.23 3 0.068 8.68 3 0.278 15.51 3 0.160 15.67 3 0.387Extruded 9.08 3 0.079 8.09 3 0.233 16.56 3 0.116 14.95 3 0.518Compression Strength 3D 47.57 3 0.609 42.82 3 1.83 Summary (psi) @ 50%Deflection Compression Modulus (psi) Vertical 59.11 3 0.161 46.63 3 1.52Horizontal 48.52 3 0.672 45.27 2 1.53 Extruded 49.67 3 0.287 46.39 30.780 Skin Thickness (inches) 0.0190 7 0.0014 0.0050 7 0.0007 ThermalStability (% change) cmd −4.0 1 N/A −4.1 1 N/A md −1.8 1 N/A −5.7 1 N/Athickness −18.1 1 N/A −27.5 1 N/A weight −3.7 1 N/A −4.3 1 N/ACompression Creep @ 1 wk. (% loss) @ 2.0 psi loading Vertical 0.55 30.262 2.14 3 0.119 Horizontal 1.49 3 0.136 1.51 3 0.110 Extruded 1.11 30.075 1.26 3 0.116 Tensile Strength (psi) cmd 29.8 5 0.496 27.1 5 1.59md 39.5 5 0.900 25.8 5 0.729 Tensile Strength (lbs force) cmd 7.23 50.191 6.50 5 0.607 md 9.55 5 0.236 6.56 5 0.194 Tensile Modulus (psi)cmd 165.0 5 9.20 178.1 5 25.3 md 241.2 5 21.9 142.1 5 7.43 TensileElongation (%) cmd 79.0 5 9.6 55.2 5 5.8 md 87.3 5 13.0 68.0 5 12.8Dynamic Cushioning (G's) Average of 2^(nd)-5^(th) Drops 1.0 psi staticload 42.0 1 N/A 43.5 1 N/A 2.0 psi static load 62.5 1 N/A 68.3 1 N/A

Compression properties, thermal stability, density, and compressioncreep are all tested according to ASTM D3575. Tensile properties aretested according to ASTM D412. Dynamic cushioning performance is testedaccording to ASTM D1596, with the densified skin oriented parallel tothe drop direction. “MD” refers to machine or extrusion direction. “CMD”refers to cross-machine or transverse direction. In general, the foamplank with one densified skin has higher compression strengths, higherthree dimensional compression strength summary, increased compressionmodulus, improved thermal stability, increased tensile strength,increased tensile modulus, increased tensile elongation, and improveddynamic cushioning performance compared to foam plank without densifiedskins.

The densified skin foam plank 50 having enhanced properties alsocompares well to laminated foam structures comprising one or more skinsof high density foam sheet laminated to a core of one or more layers oflow density foam, as detailed in U.S. Pat. Nos. 5,876,813 and 5,882,776.However, the densified skin foam plank 50 of the present invention canbe produced utilizing an in-line single step process without thehandling complexities and associated higher costs of traditionalmulti-step laminated product production processes. An illustrativesummary of properties of the foam plank sample having one densified skindetailed in Example 1, compared to a multi-density foam sheet laminate(MDL) comprising a 0.125″ thick, 6 lb/ft³ foam sheet laminated to a twoinch thick 1.7 lb/ft³ LDPE four layer foam core, is shown in Table 2below.

TABLE 2 ILLUSTRATIVE PROPERTY SUMMARY VS. MDL Example 1 MDL TESTPERFORMED With Densified Skins U.S. Pat. No. 5,876,813 Density (pcf)2.28 Skin 6.91 pcf 1.90 Cell Count (cpi) Skin Layer cmd N/A 28 md N/A 28Middle cmd 21.3 20 md 21.3 22 Compression Strengths (psi) @ 25%Deflection 50% Deflection Vertical 7.70 4.63 15.50 12.01 Horizontal 8.236.35 15.51 10.68 Extruded 9.08 9.92 16.56 13.64 Compression Strength 3DSummary (psi) @ 47.57 36.33 50% Deflection Compression Modulus (psi)Vertical 59.11 60.24 Horizontal 48.52 53.60 Extruded 49.67 145.90 SkinThickness (inches) 0.0190 0.1290 Thermal Stability (% change) cmd −4.0−0.2 md −1.8 −0.7 thick −18.1 −2.8 weight −3.7 −0.1 Comp. Creep @ 1 week(% loss) @ 2.0 psi loading Vertical 0.55 36.3 Horizontal 1.49 2.49Extruded 1.11 0.97 Tensile Strength (psi) cmd 29.8 49.3 md 39.5 62.3Tensile Strength (lbs force) cmd 7.23 11.84 md 9.55 15.25 TensileModulus (psi) cmd 165.0 381.4 md 241.2 574.0 Tensile Elongation (%) cmd79.0 63.9 md 87.3 63.3 Tensile Strength Hinge (psi) cmd 308.6 133.8 md421.6 154.6 Tensile Strength of Hinge (lbs force) cmd 2.93 8.63 md 4.019.97 Tensile Modulus Hinge (psi) cmd 2681 1383 md 3991 1662 TensileElongation of Hinge (%) cmd 24.0 28.4 md 27.0 25.0 Shear Strength ofHinge (psi) cmd 13867 6110 md 17503 6565 Shear Strength of Hinge (lbsforce) cmd 13.17 39.47 md 16.63 42.34 Modulus in Shear (psi) cmd 16354583284 md 106072 92846 Hinge Fatigue (cycles to break) cmd 100 (no break)100 (no break) md 100 (no break) 100 (no break) Dynamic Cushioning (G's)Average of 2^(nd)-5^(th) Drops 1.0 psi static load 42.0 51.3 2.0 psistatic load 62.5 90.8

All shear properties are tested according to ASTM D3163, modified forplastic foam testing. In general, both the foam plank of Example 1 withdensified skins and the multi-density foam sheet laminate (MDL) productpossess properties which make them suitable for protective packagingapplications. In particular, both products are suitable for packagingapplications containing collapsible packing members that can be rotatedabout a strong hinge point to form an integral end cap for electronicand other goods cushioning protection. One significant advantage of thedensified skin foam plank of Example 1 over the MDL product is that itpossesses superior dynamic cushioning performance, as shown at thebottom of Table 2 above.

While the present invention has been described with reference to one ormore particular embodiments, those skilled in the art will recognizethat many changes may be made thereto without departing from the spiritand scope of the present invention. For example, the heating sourcesimplemented at the heating stations may comprise virtually any type orcombination of heating sources. Additionally, thicknesses of the coreand densified skins may vary greatly. Most importantly, it is possibleto create structures similar to those described herein by densifying theoutside skins of laminated plank. Laminating together individual sheetsor rolls of ethylene polymer foam sheet using heat, adhesives, or othermeans produces laminated plank. While this would require separatehandling of the sheet and laminated plank, the resulting structure wouldbe equivalent to the present invention. In addition, it is also possibleto densify the skin of an individual foam sheet layer as it is beingproduced on a foam sheet line utilizing the techniques described herein.The densified skin foam sheet rolls or sheets can be laminated withother non-densified skin foam sheet rolls or sheets to producestructures equivalent to what is described herein. Each of theseembodiments and obvious variations thereof are contemplated as fallingwithin the spirit and scope of the claimed invention.

The following examples are provided for additional illustrativepurposes, but the invention described herein should not be consideredlimited thereto.

EXAMPLE 1

Pellets of Westlake 606 low density polyethylene (specific gravity,0.916 g/cm³; melt index, 2.3 grams/10 minutes) and Schulman F20Vcrystalline silica concentrate based in low density polyethylene are fedinto a 48:1, L/D Wilmington, 3.0 inch (76 mm) single screw extruderoperating at an average of 34 revolutions per minute and melted. A31isobutane, pressurized to 1800 psi, is added to the melted polymerthrough an injection port in the second zone of the extruder at 7.5parts per hundred by weight of polymer. Liquefied permeation modifier,Pationic 1052 (blend of mono, di and tri glycerol stearates), is alsoinjected in the second zone of the extruder at 1.0 part per hundred byweight of polymer. The isobutane blowing agent, permeation modifier, andmelted polymer are mixed and subsequently cooled to a melt temperatureof 215° F. The extruder head pressure is regulated by a Normag 2200 gearpump pressure control system. The gear pump is operating at 16.8revolutions per minute and delivers the mixed melt to a temperaturecontrolled, hydraulically pressurized piston chamber at a pressure of300 psi.

When the material filling the chamber moves the piston a predefineddistance, a limit switch activates both the drive system and the diegate system, allowing the piston to eject the material through a radialdie having a cross sectional area of 1.0 in², at a rate of 6025 lb/hr.The resulting foam plank has a fresh density of 2.25 lb/ft3, a linearcell count of 20 cells per inch, a width of 24 inches, and a thicknessof 2.375 inches. Following a stabilization period in a forming table for5 minutes, the plank is conveyed to an integral heating/edge trimmingstation.

The heating station has one heating zone comprised of two side by sideLeister model CH-6056 hot air guns, each capable of supplying 550 litersper minute of hot air at a 2 millibar back pressure. Each 220 voltLeister unit has a 3400 watt heater and a 12 inch wide nozzle with a{fraction (1/32)} inch to {fraction (3/32)} inch dogbone gap. The hotair gun nozzle discharges are positioned 3 inches above the surface of adrive belt along which the plank is fed. Therefore, the heating stationis set up for one-sided skin densification only. A pressure regulatedroller is positioned immediately after the Leister hot air nozzles at aninitial gap of 2.25 inches above the drive belt.

Plank is fed through the heating station/edge trimming assembly at aspeed of 20 feet per minute where the plank is trimmed to a width of 18inches and the initial skin densification occurs on the top side of theplank only. The discharge temperature of the heated air from the Leisterhot air nozzles is controlled at 600° F. To simulate multiple heatingzones, the plank is immediately fed back through the heating stationassembly multiple times, with the same side positioned to the top, untilno further skin densification occurs. Following a total of two passesthrough the heating station, the top skin of the densified skin plankhas a thickness of 19 mils at a density greater than 40 lb/ft³.Following a total of four passes through the heating station, the topskin of the densified skin plank has a thickness of 20 mils at a densitygreater than 45 lb/ft³. Following a total of five passes through theheating station, the top skin of the densified skin plank has athickness of 22 mils at a density greater than 50 lb/ft³.

EXAMPLE 2

Commercially available polyethylene foam plank (2.125 inches thick, 108inches in length, 24 inches wide) made by an accumulating extrusionprocess similar to what is discussed in Example 1 above, except that thedensity of the plank is 1.7 lb/ft³ and it has been colored dark blue, isfed to the heating station/edge trimming assembly described inExample 1. The plank has been aged under ambient conditions for 28 days.The belt speed and the Leister hot air nozzle temperature are varied todetermine the response of total heat input on densified skin thickness.The skin thickness of the commercial plank prior to being passed throughthe heating station is 5 mils. At a belt speed of 25 ft/min and a nozzletemperature of 600° F., a maximum skin thickness of 14 mils at a densitygreater than 40 lb/ft³ is obtained after five passes through the heatingstation. The plank surface was allowed to cool back to ambienttemperature between each pass. At a belt speed of 25 ft/min and a nozzletemperature of 425° F., a maximum skin thickness of 12 mils at a densitygreater than 40 lb/ft³ is obtained after three passes through theheating station. At a belt speed of 36 ft/min and a nozzle temperatureof 425° F., a maximum skin thickness of 10 mils at a density greaterthan 40 lb/ft³ is obtained after two passes through the heating station.At a belt speed of 36 ft/min and a nozzle temperature of 600° F., amaximum skin thickness of 13 mils at a density greater than 40 lb/ft³ isobtained after four passes through the heating station.

EXAMPLE 3

Pellets made from recycled foam originally comprising a resin blend of75% Equistar NA951 low density polyethylene (specific gravity, 0.916g/cm³; melt flow index of 2.6 grams/10 minutes) and 25% Equistar NA940low density polyethylene (specific gravity, 0.925 g/cm³; melt flow indexof 0.25 grams/10 minutes) are fed into a 48:1, L/D Wilmington, 3.0 inch(76 mm) single screw extruder operating at an average of 34 revolutionsper minute and melted. The recycled foam pellets also contain 2.0 weightpercent of a 50% talc concentrate and 1.2 weight percent of Pationic1052 permeation modifier (blend of mono, di and tri glycerol stearates).A31 isobutane, pressurized to 1800 psi, is added to the melted polymerthrough an injection port in the second zone of the extruder at 11.1parts per hundred by weight of polymer. The isobutane blowing agent andmelted polymer are mixed and subsequently cooled to a melt temperatureof 215° F. The extruder head pressure is regulated by a Normag 2200 gearpump pressure control system. The gear pump is operating at 16.8revolutions per minute and delivers the mixed melt to a temperaturecontrolled, hydraulically pressurized piston chamber at a pressure of400 psi.

When the material filling the chamber moves the piston a predefineddistance, a limit switch activates both the drive system and the diegate system, allowing the piston to eject the material through a radialdie having a cross sectional area of 1.0 in², at a rate of 7100 lb/hr.The resulting foam plank has a fresh density of 1.39 lb/ft3, a linearcell count of 22 cells per inch, a width of 24 inches, and a thicknessof 2.5 inches. Following a stabilization period in a forming table for 4minutes, the plank is conveyed to an integral heating/edge trimmingstation as described in Example 1.

Plank is fed through the heating station/edge trimming assembly at aspeed of 20 feet per minute where the plank is trimmed to a width of 18inches and the initial skin densification occurs on the top side of theplank only. The discharge temperature of the heated air from the Leisterhot air nozzles is controlled at 600° F. The plank is immediately fedback through the heating station assembly multiple times, with the sameside positioned to the top, until no further skin densification occurs.Average total plank thickness is recorded initially and following eachpass through the heating station assembly as a measure of the skindensification. Following three passes through the heating stationassembly, the average plank thickness is 2.25 inches, equating to adensified skin thickness of 12.5 mils at a skin density greater than 50lb/ft³.

EXAMPLE 4

Densified foam plank is prepared according to Example 3 except theclearance between the heated air nozzles and the driven conveyor belthas been reduced to 2.75 inches (0.25 inch reduction) putting the nozzledischarge within 0.25 inches of the plank surface during the first passthrough the heating station. Following three passes through the heatingstation assembly, the average plank thickness is 2.20 inches, equatingto a densified skin thickness of 13.5 mils at a skin density greaterthan 50 lb/ft³.

EXAMPLE 5

Densified foam plank is prepared according to Example 4 except theclearance between the nip roller and the driven conveyor belt has beenreduced to 2.00 inches (0.25 inch reduction) thereby increasing thepressure on the foam as it exits the area immediately adjacent to theheated air nozzles. Following three passes through the heating stationassembly, the average plank thickness is 2.125 inches, equating to adensified skin thickness of 16.5 mils at a skin density greater than 50lb/ft³.

What is claimed is:
 1. A method for making a densified skin polyolefinicfoam plank comprising the steps of: extruding said foam plank from anextruder; stabilizing said foam plank by holding for at least about 30seconds after said extruding step; heating an outer surface of saidstabilized foam to collapse and melt said cells adjacent to said outersurface to form a densified skin, said densified skin having a densitygreater than a density of said foam before said heating.
 2. The methodof claim 1 wherein said foam is a low density ethylene polymer foam. 3.The method of claim 1 wherein said foam has a density from about 0.9lb/ft³ to about 5.0 lb/ft³.
 4. The method of claim 1 wherein said foamis a dimensionally stable closed cell foam.
 5. The method of claim 1wherein said foam has a thickness greater than 1.0 inch.
 6. The methodin claim 1 wherein said densified skin has a thickness from about 12.5mils to about 100 mils.
 7. The method of claim 2 further including thestep of mixing and melting a polymer mixture comprising at least 50weight percent low density polyethylene (LDPE), mixing with one or moreblowing agents, cooling and pressurizing said mixture prior to extrudingsaid low density ethylene polymer foam.
 8. The method of claim 1 whereinsaid foam is held for at least 2 to 5 minutes after said extruding stepbefore said heating step.
 9. A method for making a densified skinpolyolefinic foam plank comprising the steps of; extruding a low densityethylene polymer foam; after said foam has been held for at least about30 seconds after extruding said foam for stabilizing said foam, heatingat least one outer surface of said low density ethylene polymer foam toform a densified skin on said outer surface, said densified skin havinga density ranging between ten times greater than a density of said lowdensity ethylene polymer foam before heating and approximately equal toa density of an ethylene polymer used to produce said foam.
 10. Themethod of claim 9 wherein said low density ethylene polymer foam has adensity from about 1.2 lb/ft³ to about 4.0 lb/ft³.
 11. The method ofclaim 9 wherein said low density ethylene polymer foam has a thicknessgreater than 1.0 inch.
 12. The method in claim 9 wherein said densifiedskin has a thickness from about 12.5 mils to about 100 mils.
 13. Themethod of claim 9 further including the step of mixing and melting apolymer mixture comprising at least 50 weight percent low densitypolyethylene (LDPE), mixing with one or more blowing agents, cooling andpressurizing said mixture prior to extruding said low density ethylenepolymer foam.
 14. The method of claim 13 wherein said low densityethylene polymer foam contains from 0 to 40 percent of an ethylenecopolymer.
 15. The method of claim 13 wherein said blowing agent is anorganic blowing agent.
 16. The method of claim 15 wherein said blowingagent is isobutane, n-butane, or a blend thereof.
 17. The method ofclaim 9 wherein said low density ethylene polymer foam is stabilizedbefore said heating step.
 18. The method of claim 9 wherein said lowdensity ethylene polymer foam is held after said extruding step for aperiod of time ranging from approximately thirty seconds toapproximately five minutes prior to said heating step.
 19. The method ofclaim 9, wherein said low density ethylene polymer foam is held for atleast 2 to 5 minutes after extruding said foam before said heating step.